Frequency converting circuit, signal processing circuit and receiver

ABSTRACT

An example frequency converting circuit generates a multiplied signal obtained by multiplying a local signal by an amplified signal generated by an amplifying portion. The frequency converting circuit includes a converter which converts the amplified signal into a current signal and a switching circuit which multiplies the current signal by the local signal and generates the multiplied signal. An impedance element supplies a first direct current from the amplifier and a second direct current from the switching circuit to the converter.

CROSS-REFERENCE TO RELATED APPLICATION

The present invention claims the benefit of the priority application ofJapanese Patent Application No. 2010-153151, filed on Jul. 5, 2010. Thisapplication is hereby incorporated by reference in its entirety.

FIELD

One embodiment relates to a frequency converting circuit, a signalprocessing circuit and a receiver.

BACKGROUND

When the amplitude of an input signal is small, the frequency converteris susceptible to the influence of noise and has a problem that it isnot possible to correctly convert the frequency of the input signal.

Conventionally known frequency converters additionally input a currentin a voltage-current converter to provide the difference between theamount of a direct current which flows into a switching step and theamount of a current which flows into the voltage-current converter.Consequently, it is possible to reduce noise that is produced in theswitching step and to operate the switching step even with a localsignal having small amplitude.

However, the conventional frequency converters additionally supply thecurrent to the voltage-current converter, and therefore increase powerconsumption.

Therefore the present invention provides a frequency converting circuit,a signal processing circuit and a receiver which can convert thefrequency even if the amplitude of a local signal is small and which cansuppress an increase of power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view illustrating a signal processing circuit according toa first embodiment;

FIG. 1B is a circuit diagram illustrating an example of an amplifieraccording to the first embodiment;

FIG. 1C is a view illustrating an example of an impedance elementaccording to the first embodiment;

FIG. 1D is a view illustrating an example of a current-voltageconverting circuit;

FIG. 2A is a view illustrating a signal processing circuit according toa second embodiment;

FIG. 2B is a view illustrating an example of a controlling portion;

FIG. 3 is a view illustrating a signal processing circuit according to athird embodiment;

FIG. 4A is a view illustrating a signal processing circuit according toa fourth embodiment;

FIG. 4B is a view illustrating an example of an amplifier according tothe fourth embodiment;

FIG. 4C is a view illustrating an example of an impedance elementaccording to the fourth embodiment;

FIG. 5 is a view illustrating a signal processing circuit according to afifth embodiment;

FIG. 6 is a view illustrating a signal processing circuit according to afirst modified example of the fifth embodiment;

FIG. 7 is a view illustrating a signal processing circuit according to asecond modified example of the fifth embodiment;

FIG. 8 is a view illustrating a signal processing circuit according to athird modified example of the fifth embodiment;

FIG. 9 is a view illustrating a signal processing circuit according to afourth modified example of the fifth embodiment; and

FIG. 10 is a view illustrating a receiver according to a sixthembodiment.

DETAILED DESCRIPTION

According to one embodiment, a frequency converting circuit whichgenerates a multiplied signal obtained by multiplying a local signal byan amplified signal generated by an amplifying portion comprising afirst transistor having a drain terminal connected to a first powersource potential, the frequency converting circuit comprising: aconverter which comprises a second transistor of which gate terminal isconnected to the amplifying portion and which converts the amplifiedsignal inputted to the gate terminal into a current signal; a switchingcircuit which comprises two third-transistors of which a source terminalis connected each other and which multiplies the current signal by thelocal signal and generates the multiplied signal; and an impedanceelement which comprises a first terminal connected to a source terminalof the first transistor, a second terminal connected to a drain terminalof the second transistor and a third terminal connected to the sourceterminal of the third transistor, which inputs a first direct currentinputted from the source terminal of the first transistor and a seconddirect current inputted from the source terminal of the third transistorinto the drain terminal of the second transistor, of which impedance isan ACwise high impedance between the first terminal and the secondterminal.

Hereinafter, embodiments of the present invention will be described withreference to the drawings. In each drawing, the same components aredesignated by the same reference numerals, and descriptions thereof willnot be repeated.

<First Embodiment>

FIG. 1A is a circuit diagram of a signal processing circuit 100according to the first embodiment. The signal processing circuit 100illustrated in FIG. 1A has an amplifying portion 110 which outputs anamplified signal obtained by amplifying an input signal, and a frequencyconverting circuit 120 which outputs a multiplied voltage signalobtained by converting the frequency of the amplified signal.

The amplifying portion 110 has an amplifier 111. FIG. 1B is a circuitdiagram illustrating an example of the amplifier 111. The amplifier 111has a transistor M4. A drain terminal of the transistor M4 is connectedto a first terminal which has a first power source potential (Vdd)through an inductance element L1. A source terminal of the transistor M4is connected to a second terminal b. The source terminal of thetransistor M4 and second terminal b are DCwise short-circuited. Further,the source terminal of the transistor M4 is connected to a groundthrough a capacitor C2, and is thereby ACwise grounded. The gateterminal of the transistor M4 is connected to a third terminal c.Between the gate terminal of the transistor M4 and third terminal c, aresistance R is provided having one end connected to the third terminalc and gate terminal of the transistor M4 and the other end applied to abias voltage V_(BIAS). Further, the drain terminal of the transistor M4is connected to a fourth terminal d through the capacitor C1 providedbetween the inductance element L1 and drain terminal of the transistorM4.

Currents flowing into the amplifier 111 will be described in terms of adirect current and alternating current, respectively.

First, the alternating current flowing into the amplifier 111 will bedescribed.

The amplifier 111 amplifies an input signal f(ω₁) which is analternating current signal, and generates an amplified signal G₁f(ω₁)which is an alternating current signal. The amplifier 111 amplifies theinput signal f(ω₁) inputted from the third terminal c to generate theamplified signal G₁f(ω₁), and outputs the amplified signal from thefourth terminal d. Here, G₁ is gain of the amplifier 111. The amplifiedsignal G₁f(ω₁) outputted from the fourth terminal d is inputted in thefrequency converting circuit 120.

Next, the direct current flowing into the amplifier 111 will bedescribed.

The amplifier 111 receives an input of a direct current I_(mixer)required to drive the amplifier 111 at the first terminal a, and outputsthe direct current from the second terminal b. The direct currentI_(mixer) outputted from the second terminal b is inputted to thefrequency converting circuit 120.

Referring back to FIG. 1A, the frequency converting circuit 120 has aconverter 124, an impedance element 121, a switching circuit 122 and acurrent-voltage converting circuit 123.

The converter 124 is a circuit which converts the inputted amplifiedsignal G₁f(ω₁) from the voltage into the current, and generates acurrent signal. Hence, the converter 124 will also be referred to as“voltage-current converter 124” hereafter.

The voltage-current converter 124 has a transistor M1. A gate terminalof the transistor M1 is connected to the fourth terminal d of theamplifier 111. A source terminal of the transistor M1 is grounded, andthe drain terminal is connected to a terminal j of the impedance element121.

The impedance element 121 has a terminal i connected to the terminal bof the amplifier 111, a terminal h connected to the switching circuit122 and the terminal j connected to the voltage-current converter 124.The impedance element 121 provides a high AC impedance and provides alow DC impedance between the terminals h and i and between the terminalsi and j. That is, the impedance element 121 connects the source terminalof the amplifier 111 and frequency converting circuit 120 with a high ACimpedance and low DC impedance. With this connection, the current Imixerused to drive the amplifier 111 of the amplifying portion 110 issupplied to the voltage-current converter 124 of the frequencyconverting circuit 120 through the impedance element 121.

By contrast, a high AC impedance is provided between the terminals h andi and between the terminals i and j, and therefore an alternatingcurrent signal (for example, the above-mentioned current signal) whichis an alternating current signal flowing into the frequency convertingcircuit 120 and which flows between the terminals h and j is preventedfrom flowing outside the frequency converting circuit 120 (amplifier111) through the terminal i. Further, the alternating current signal isalso prevented from flowing into the frequency converting circuit 120from outside (amplifier 111) through the terminal i. The AC and DCimpedances are both low between the terminals h and j. That is, theimpedance element 121 connects the switching circuit 122 andvoltage-current converter 124 with both low AC and DC impedances.

Further, when seen from the terminal i, the impedance element 121operates as if the impedance element 121 is ACwise grounded. That is,when seen from the source terminal of the transistor M4 of the amplifier111, the impedance element 121 is ACwise grounded. With this connection,it is possible to adjust the source terminal of the transistor M4 of theamplifier 111 to a constant potential (ground in the presentembodiment), and keep the constant potential between the drain terminaland source terminal of the transistor M4 of the amplifier 111.

FIG. 1C is a circuit diagram illustrating an example of the impedanceelement 121. The terminals h, i and j in FIG. 1C respectively correspondto the terminals h, i and j in FIG. 1A.

The impedance element 121 illustrated in FIG. 1C has an inductanceelement L2 having one end connected to the terminal i and having theother end connected to the terminal h and terminal j, and a capacitor C3having the one end connected to the terminal i and having the other endgrounded. That is, the impedance element 121 in FIG. 1C directlyconnects the switching element 122 and voltage-current converter 124,and connects the amplifying portion 110 and switching circuit 122, andthe switching circuit 122 and voltage-current converter 124 through theinductance element L2. By this means, it is possible to provide both lowAC and DC impedances between the terminals h and j, and provide a highAC impedance and a low DC impedance between the terminals i and j.

Further, the terminal i is grounded through the capacitor C3. Thecapacitor C3 has a low AC impedance and high DC impedance. Thus, theterminal i is AC grounded.

The switching circuit 122 has two transistors M2 and M3, and outputs amultiplied signal by multiplying an amplified signal with a local signal(alternating current signal). As described later, the switching circuit122 performs a switching operation of passing or blocking the currentaccording to the magnitude of the given local signal.

The two transistors M2 and M3 of the switching circuit 122 are alignedin parallel. The gate terminals of the transistors M2 and M3 receivelocal signals −g(ω₂) and g(ω₂), respectively. The source terminals ofthe transistors M2 and M3 are connected to the terminal h of theimpedance element 121. The drain terminals of the transistors M2 and M3are connected to terminals f and g of the current-voltage convertingcircuit 123, respectively.

The current-voltage converting circuit 123 converts the multipliedsignal (current) outputted from the switching circuit 122 into thevoltage, and outputs the multiplied voltage signal. The terminal e ofthe current-voltage converting circuit 123 is connected to the firstpower source potential (Vdd). The multiplied voltage signals areoutputted from the terminal f and terminal g of the current-voltageconverting circuit 123.

FIG. 1D illustrates an example of the current-voltage converting circuit123. The current-voltage converting circuit 123 in FIG. 1D has currentsources 123 a and 123 b. Both of the current sources 123 a and 123 bhave one ends connected to the terminal e. The other end of the currentsource 123 a is connected to the terminal f. The other end of thecurrent source 123 b is connected to the terminal g. The terminals e, fand g respectively correspond to the terminals e, f and g in FIG. 1A.

Next, signals flowing into the frequency converting circuit 120 will bedescribed in terms of a direct current signal and alternating currentsignal using FIG. 1A.

First, the alternating current signal flowing into the frequencyconverting circuit 120 will be described.

The amplified signal G₁ f(ω₁) amplified by the amplifying portion 110 isinputted to the voltage-current converter 124. The voltage-currentconverter 124 converts the amplified signal G₁f(ω₁) into the currentsignal G_(M1)G₁f(ω₁). Here, G_(M1) is a conversion gain of thetransistor M1 (voltage-current converter 124). The current signalG_(M1)G₁f(ω₁) is inputted in the switching circuit 122 through theimpedance element 121. The transistors M2 and M3 of the switchingcircuit 122 pass the inputted current signal G₁ f(ω) between the drainand source when the potentials of the gate given from the respectivelocal signals −g(ω₂) and g(ω₂) are high.

By contrast, the transistors M2 and M3 block the current between thedrain and source when the potentials of the gate are low. By this means,the current signal G_(M1)G₁ f(ω) is multiplied by local signals g(ω₂)and −g(ω₂) to generate a multiplied signal. The multiplied signaloutputted from the switching circuit 122 is converted into a multipliedvoltage signal by the current-voltage converting circuit 123. Themultiplied voltage signal which is the final output signal of thefrequency converting circuit 120 is represented by 2G₁G₂ f(ω₁)×g(ω₂)(=G₁G₂ f(ω1)×g(ω₂)−[−G₁G₂ f(ω₁)×g(ω₂)]). G₂ is the final frequencyconversion gain of the frequency converting circuit 120.

The above-described amplified signal, current signal, local signal,multiplied signal, and multiplied voltage signal are all alternatingcurrent signals. In this way, the alternating current amplified signalinputted in the frequency converting circuit 120 is multiplied by thelocal signal and the resultant is outputted.

Next, a direct current flowing into the frequency converting circuit 120will be described.

Direct currents flow into the transistors M2 and M3 of the switchingcircuit 122, respectively. The amount of the direct current flowing intoone of these transistors M2 and M3 are represented by ½*I_(SW). That is,the total amount of the direct currents flowing into the switchingcircuit 122 is represented by I_(SW).

This direct current I_(SW) is also inputted in the voltage-currentconverter 124 connected in series through the impedance element 121. Inthe voltage-current converter 124, the direct current I_(mixer) consumedto drive the amplifier 111 further flows through the impedance element121. Hence, the direct current flowing into the voltage-currentconverter 124 is represented by I_(SW)+I_(mixer).

Thus, the direct current flowing into the voltage-current converter 124is greater, by the amount of the direct current I_(mixer) inputted fromthe amplifier 111, than the direct current flowing in the switchingcircuit 122. Thus, providing the difference between the total amounts ofdirect currents flowing into the voltage-current converter 124 ispreferable for circuit characteristics of a frequency convertingcircuit. This reason will be described using FIG. 1A.

In many cases, the voltage-current converter 124 is required to linearlyconvert the amplified signal G f(ω) which is an inputted voltage signalinto a current signal. Therefore, there is a desired value of a directcurrent applied in advance between the drain and source of thetransistor M1 of the voltage-current converter 124. By contrast, thedirect currents to be applied between the drains and sources of thetransistors M2 and M3 of the switching circuit 122 are preferablysmaller than the desired value of the voltage-current converter 124.When the direct current flowing into the switching circuit 122 is small,the demand for the voltage amplitude of the local signal g is relaxedand a desired switching operation can be realized even if the amplitudeis small. It is also possible to suppress thermal noise caused by thedirect currents flowing between the drains and sources of thetransistors M2 and M3 of the switching circuit 122.

By causing the total amount of the direct current flowing between thedrain and source of the transistor M1 of the voltage-current converter124 to be larger than the total amount of the direct currents flowingbetween the drains and sources of the transistors M2 and M3 of theswitching circuit 122, it is possible not only to adjust to a desiredvalue the direct current flowing between the drain and source of thetransistor M1 of the voltage-current converter 124, but also to reducethe direct currents flowing between the drain and source of thetransistors M2 and M3 of the switching circuit 122.

Consequently, by causing the total amount of the direct current flowingbetween the drain and source of the transistor M1 of the voltage-currentconverter 124 to be larger than the total amount of the direct currentsflowing between the drains and sources of the transistors M2 and M3 ofthe switching circuit 122, it is possible not only to perform linearconversion in the voltage-current converter 124, but also to reduce theamplitude of the local signal of the switching circuit 122 and suppressnoise. That is, the characteristics of the entire frequency conversioncircuit 120 are improved.

With the present embodiment, by supplying the direct current I_(mixer)used to drive the amplifying portion 110 to the voltage-currentconverter 124 through the impedance element 121, the differencecorresponding to I_(mixer) is provided between the total amount of thedirect current flowing between the drain and source of the transistor M1of the voltage-current converter 124 and the total amount of the directcurrent flowing between the drains and sources in the switching circuit122. That is, the current which is used to drive the amplifier 111 isapplied to improve characteristics of the frequency converting circuit120. By so doing, it is possible to improve the characteristics of thefrequency converting circuit 120 without providing an additional currentsource in the frequency converting circuit 120.

As described above, the frequency converting circuit and signalprocessing circuit of the present embodiment can provide a frequencyconverting circuit which can convert the frequency even if the amplitudeof a local signal is small and can suppress an increase of powerconsumption. Further, the frequency converting circuit and signalprocessing circuit can improve the conversion gain and noisecharacteristics at the same time.

In addition, although the terminal i is configured to be groundedthrough the capacitor C3 having a low AC impedance with the presentembodiment, the terminal i does not need to be grounded. The terminal bof the amplifying portion only needs to be connected to a portion havinga constant potential (no fluctuation in the potential) through anelement having a low AC impedance. That is, for example, the terminal bonly needs to be AC grounded.

<Second Embodiment>

Next, signal processing circuit 200 according to a second embodiment ofthe present invention will be described. FIG. 2A is a circuit diagram ofthe signal processing circuit 200 according to the second embodiment.The signal processing circuit 200 employs a configuration in which acontrolling portion 210 is further added to the signal processingcircuit according to the first embodiment. The other configurations arethe same as those in FIG. 1. In addition, although the controllingportion 210 is configured to be provided outside the frequencyconverting circuit 120, the controlling portion 210 may be configured tobe provided inside the frequency converting circuit 120.

The controlling portion 210 has terminals k and l which receive inputsof local signals alternating current signals). The controlling portion210 has a terminal o which receives an input of a reference voltageV_(REF) (direct current signal) which is given from the outside. Thecontrolling portion 210 has a terminal m and a terminal n which outputthe inputted local signals as they are.

The controlling portion 210 monitors the first potential through theterminal p connected to, for example, a connecting portion Q between theterminal b of the amplifier 111 and the terminal i of the impedanceelement 121. The controlling portion 210 compares the reference voltageV_(REF) and the first potential of the connecting portion Q, andsuperimposes the direct current voltages from the terminals m and n on alocal signal such that the first potential of the connecting portion Qand the reference voltage match, and outputs the direct voltages. Thatis, the controlling portion 210 inputs the signal, that is obtained bysuperimposing the local signal and direct current voltage, to thetransistor M3 through the terminal m and to the transistor M2 throughthe terminal n. The reference voltage is a potential required to causethe amplifier 111 to perform a desired operation such as linearamplification at a desired amplification factor, and is determined inadvance. The reference voltage is designed such that the rate of themagnitude of the direct current flowing into the voltage-currentconverter 124 to the magnitude of the direct current flowing into theswitching current 122 becomes a desired value.

The direct current voltage outputted from the controlling portion 210adjusts the total amount I_(SW) of the direct current flowing into theswitching circuit 122 by controlling the voltages of the respective gateterminals of the transistor M2 and M3 of the switching circuit 122. Thecontrolling portion 210 controls the current flowing into thevoltage-current converter 124 by adjusting the direct current I_(SW).Further, the controlling portion 210 controls the first potential of theconnecting portion Q by adjusting the direct current I_(SW).Hereinafter, the reason why the first potential of the connectingportion Q can be controlled by adjusting the direct current I_(SW) bythe controlling portion 210 will be described.

The voltage-current converter 124 receives an input of a current I_(VI)of the sum of the direct current I_(SW) and the direct current I_(mixer)inputted from the amplifying portion 111. The first potential of theconnecting portion Q is determined based on the sum of the voltage dropbetween the terminals i and j of the impedance element 121 and thevoltage drop of the voltage-current converter 124. As described above,the voltage drop between the terminals i and j of the impedance element121 is DCwise very small. Hence, if the voltage drop between theterminals i and j of the impedance element 121 is neglected, thepotential of the connecting portion Q is determined based on the voltagedrop of the voltage-current converter 124. The voltage drop of thevoltage-current converter 124 is determined based on the characteristicsof the transistor M1 and the current I_(VI) flowing into the transistorM1. As described above, the current I_(VI) is represented byI_(mixer)+I_(SW). The direct current I_(mixer) is determined accordingto the first potential of the connecting portion Q. Although the directcurrent I_(mixer) cannot be controlled directly, the value of I_(mixer)can be controlled by adjusting I_(SW). Consequently, the current I_(VI)is changed by adjusting the direct current I_(SW). By this means, it ispossible to control the first potential of the connecting portion Q.

Further, when the potential of the connecting portion Q is finallydetermined, the direct current I_(mixer) flowing into the amplifier 111is determined. When the potential of the connecting portion Q becomes adesired value, the rate of the total amount I_(SW) of the direct currentflowing into the switching circuit 122 to the direct currentI_(VI)=I_(SW)+I_(mixer) flowing into the voltage-current converter 124becomes a desired value.

As described above, the operation points of the all circuits are fixedby providing the controlling portion 210. By this means, even if themanufacturing state, environmental temperature and power source voltageof circuits fluctuate, the bias of the switching circuit 122 isautomatically adjusted such that the connecting portion Q is kept at adesired potential and the desired direct current I_(mixer) flows intothe amplifier 111. Further, by adjusting the direct current voltageinputted to the gate terminals of the transistors M2 and M3, it ispossible to determine the current I_(SW) flowing into the switchingcircuit 122 irrespectively of the sizes of the transistors M2 and M3,the manufacturing state, or the environmental temperature of thecircuits.

Next, an example of a detailed configuration of the controlling portion210 will be described.

The controlling portion 210 illustrated in FIG. 2B has a controllingcircuit 230 and a reference voltage generating circuit 220. In addition,although the controlling portion 210 employs a configuration includingthe reference voltage generating circuit 220 in FIG. 2B, the controllingportion 210 may be configured to be provided outside the signalprocessing circuit 200 as illustrated in FIG. 2A and input the referencevoltage in the controlling portion 210.

The reference voltage generating circuit 220 has a MOS transistor 221and a current source 222, and generates the reference voltage V_(REF).The drain terminal and gate terminal of the MOS transistor 221 areconnected to the first power source potential Vdd. The source terminalof the MOS transistor 221 is connected to one end of the current source222. The other end of the current source 222 is connected to the secondpower source potential (ground in FIG. 2B). The source terminal of theMOS transistor 221 and one end of the current source 222 are connectedto the terminal o of the controlling circuit 230. The reference voltagegenerating circuit 220 outputs the reference voltage V_(REF) from thesource terminal of the MOS transistor 221.

By adjusting the size of the MOS transistor 221 and the condition of thecurrent amount of the current source 222, the reference voltagegenerating circuit 220 reproduces desired states of the direct currentvoltages I_(mixer) and I_(VI) of the amplifier 111 and voltage-currentconverter 124. That is, the reference voltage generating circuit 220operates as a dummy for the amplifier 111 and voltage-current converter124. The reference voltage generating circuit 230 outputs the firstvoltage of the connecting portion Q in FIG. 2 when the amplifier 111 andvoltage-current converter 124 are operating in ideal states.

The controlling circuit 230 has resistances 231-1 and 231-2 and acomputing amplifier 232. The terminal o of the computing amplifier 232is connected with the source of the MOS transistor 221 and one end ofthe current source 222. The terminal p of the computing amplifier 232 isconnected to the connecting portion Q in FIG. 2A. The output terminal ofthe computing amplifier 232 is connected to the terminals k and l andthe terminals m and n through the resistances 231-1 and 231-2. Inaddition, the terminal k and terminal m, and the terminal I and terminaln are short-circuited, respectively.

The terminal o of the computing amplifier 232 receives an input of thereference voltage V_(REF), and the terminal p receives an input of thefirst voltage of the connecting portion Q in FIG. 2A.

The terminals k and l of the controlling portion 210 receive an input ofa local signal from outside. The terminals m and n of the controllingportion 210 output signals obtained by superimposing the direct currentvoltage outputted from the computing amplifier 232 and local signal.

The controlling portion 210 has a feedback route reaching the terminal pfrom the terminals m and n through the switching circuit 122 andimpedance element 121. When the potential of the connecting portion Q isdifferent from the reference voltage V_(REF), according to the virtualshort-circuiting effect of the computing amplifier 232, the directcurrent voltage levels of the terminals in and n are adjusted such thatthe potential of the connecting portion Q and the reference voltageV_(REF) become equal, so that the direct current I_(SW) changes and, asa result, the potential of the connecting portion Q becomes a desiredvalue.

The frequency converting circuit 120 and signal processing circuit 200according to the present embodiment can achieve the same effect as inthe first embodiment. That is, the frequency converting circuit 120 andsignal processing circuit 200 according to the present embodiment canconvert the frequency even if the amplitude of a local signal is small,suppress an increase of power consumption and improve the gainconversion and noise characteristics.

Further, the frequency converting circuit 120 and signal processingcircuit 200 according to the present embodiment can adjust to a desiredvalue the first potential at the connecting portion Q which connects theamplifier 111 and the voltage-current converter 124 through theimpedance element 121, and operate the amplifier 111 within a desiredrange (the range where the amplifier 111 linearly operates at a desiredamplification factor).

It is possible to perform a control action such that the rate of themagnitude of the direct current I_(VI) flowing into the voltage-currentconverter 124 to the magnitude of the direct current I_(SW) flowing intothe switching circuit 122 becomes a desired value. this means, it ispossible to adjust to a desired value of the magnitude of the directcurrent I_(VI) flowing into the voltage-current converter 124, andreduce the direct current I_(SW) flowing into the switching circuit 122.As a result, the voltage-current converter 124 can linearly convert anamplified signal, which is an inputted voltage signal, into a currentsignal. In addition, the switching circuit 122 can decrease the voltageamplitude of a local signal and reduce thermal noise of the switchingcircuit 122 resulting from the direct currents flowing between thesource terminals and drain terminals of the transistors M2 and M3 of theswitching circuit 122.

<Third Embodiment>

Next, a signal processing circuit 300 according to a third embodimentwill be described. FIG. 3 is a circuit diagram of the signal processingcircuit 300. The signal processing circuit 300 has an amplifying portion310 and the frequency converting circuit 120. The signal processingcircuit 300 differs from the signal processing circuit 100 according tothe first embodiment in that the amplifying portion 310 has twoamplifiers 111-1 and 111-2.

Each configuration of the amplifiers 111-1 and 111-2 is the same as theconfiguration of the amplifier 111 described in the first embodiment.

The source terminal of the transistor M4 of the amplifier 111-1 and thedrain terminal of the transistor M4 of the amplifier 111-2 are DCwiseconnected. Further, the source terminal of the amplifier 111-1 and thedrain terminal of the amplifier 111-2 are connected to the impedanceelement 121 of the frequency converting circuit 120.

The drain terminal of the amplifier 111-1 is connected to the firstpower source potential Vdd, and the source terminal of the amplifier111-2 is connected to the second power source potential (ground).

The input terminal c of the amplifier 111-1 receives an input of aninput signal f(ω₁) which is an alternating current signal. The outputterminal d of the amplifier 111-1 is connected to the input terminal cof the amplifier 111-2. That is, the amplifier 111-1 and amplifier 111-2are connected in cascade in terms of high frequency small signalprocessing. The output terminal of the amplifier 111-2 is connected tothe gate terminal of the voltage-current converter 124 of the frequencyconverting circuit 120.

The flow of currents of the direct current and alternating current inthe signal processing circuit 300 according to the present embodimentwill be respectively described below.

First, an alternating current signal will be described.

The amplifier 111-1 receives an input of the input signal which is analternating current signal. The input signal is inputted to the inputterminal of the amplifier 111-1, is amplified by the amplifier 111-1,and is converted into a first amplified signal. The first amplifiedsignal is inputted to the amplifier 111-2, is amplified by the amplifier111-2, and is converted into a second amplified signal. The secondamplified signal is inputted to the gate terminal of the transistor M1of the voltage-current converter 124 of the frequency converting circuit120 as the output signal of an amplifying portion 310.

Next, the direct current signal will be described.

The direct current I_(amp)+I_(mixer) flows between the source terminaland drain terminal of the transistor M4 of the amplifier 111-1. Theamplifier 111-1 is driven by the direct current I_(amp)+I_(mixer).I_(amp) of the direct current I_(amp)+I_(mixer) flows between the sourceterminal and drain terminal of the transistor M4 of the amplifier 111-2.The amplifier 111-2 is driven by I_(amp). The other I_(mixer) of thedirect current I_(amp)+I_(mixer) is supplied to the voltage-currentconverter 124 through the impedance element 121.

As described above, the signal processing circuit 300 reduces powerconsumption by using the current having been used to drive the amplifier111-1 to drive the amplifier 111-2 at a later stage. Further, similar tothe first embodiment, by applying for the frequency converter 124 thecurrent used to drive the amplifier 111-1, it is possible to provide acircuit which improves performance of a frequency converting circuitwithout additional power consumption.

<Fourth Embodiment>

Next, a signal processing circuit 400 according to the fourth embodimentwill be described. FIG. 4A is a circuit diagram illustrating the signalprocessing circuit 400, and FIG. 4B is a view illustrating an example ofan amplifier 411-1 of an amplifying portion 410 of the signal processingcircuit 400. The signal processing circuit 400 according to the fourthembodiment is configured as a differential circuit using theconfiguration in which the controlling portion 210 illustrated in FIG. 2is added to the signal processing circuit 300 illustrated in FIG. 3.

The signal processing circuit 400 will be described below mainly basedon the difference from the configuration when a single-phase circuit isconfigured as a differential circuit.

The signal processing circuit 400 illustrated in FIG. 4A has theamplifying portion 410 and a frequency converting circuit 420. Theamplifying portion 410 has amplifiers 411-1 and 411-2.

The amplifier 411-1 receives an input of a differential signal (normalphase and reverse phase). FIG. 4B illustrates an example of theamplifier 411-1.

The amplifier 411-1 has a transistor M4-1 in which the gate terminal isinputted to an input terminal c-1, the drain terminal is connected to aterminal a through an inductance element L11, and the source terminal isconnected to a terminal b. The amplifier 411-1 also has a transistorM4-2 in which the gate terminal is inputted to an input terminal c-2,the drain terminal is connected to the terminal a through an inductanceelement L12, and the source terminal is connected to the terminal b.

The terminal d-1 is connected between the inductance element L11 andtransistor M4-1 through a capacitor C11. A terminal d-2 is connectedbetween the inductance element L12 and transistor M4-2 through acapacitor C12. Further, the amplifier 411-1 has a resistance R1 havingone terminal connected to the terminal c-1 and gate terminal of thetransistor M4-1 and having the other end applied the bias voltageV_(BIAS), and a resistance R2 having one end connected to the terminalc-2 and gate terminal of the transistor M4-2 and having the otherterminal applied the bias voltage V_(BIAS).

The amplifier 411-1 receives an input of differential signals from theinput terminals c-1 and c-2. The normal phase signal of the differentialsignals is inputted to the input terminal c-1 and the reverse phasesignal is inputted to the input terminal c-2. The transistor M4-1amplifies the normal phase signal, and outputs a first normal phaseamplified signal from the terminal d-1. The transistor M4-2 amplifiesthe reverse signal of input signals, and outputs the first reverse phaseamplified signal from the terminal d-2. The first normal phase amplifiedsignal and first reverse phase amplified signal are collectivelyreferred to as “first amplified signal.”

The configuration of the amplifier 411-2 is the same as theconfiguration of the amplifier 411-1. The amplifier 411-2 amplifies theinputted first amplified signal, and outputs the second normal phaseamplified signal and second reverse phase amplified signal (hereinafterreferred to as “second amplified signal”).

The terminal b of the amplifier 411-1 is connected with each sourceterminal of the transistors M4-1 and M4-2, and is thereby ACwisegrounded. The terminal a of the amplifier 411-2 is connected with theterminal b of the amplifier 411-1, and is thereby ACwise grounded.

Next, the configuration of the frequency converting circuit 420 will bedescribed. The frequency converting circuit 420 has a voltage-currentconverter 424, an impedance element 421, a switching circuit 422, andthe current-voltage converting circuit 123.

The voltage-current converter 424 has two transistors M1-1 and M1-2. Thetransistor M1-1 converts the second normal phase amplified signal into acurrent signal, and outputs a normal phase current signal. Thetransistor M1-2 converts the second reverse phase amplified signal intoa current signal, and outputs a reverse phase current signal. The normalphase current signal and reverse phase current signal will becollectively referred to as “current signal.”

The switching circuit 422 has four transistors M2-1, M2-2, M3-1, andM3-2.

The transistors M2-1 and M3-1 receive inputs of normal phase currentsignals from the source terminals, and receive inputs of normal phaselocal signals from the gate terminals. The transistors M2-2 and M3-2receive inputs of reverse phase current signals from the sourceterminals, and receive inputs of reverse phase local signals from thegate terminals. The transistors M2-1 and M3-1 multiply normal phasecurrent signals by normal phase local signals, and generate normal phasemultiplied signals. The transistors M2-2 and M3-2 multiply reverse phasecurrent signals by reverse phase local signals, and generate reversephase multiplied signals. The normal phase multiplied signal and reversephase multiplied signal will be collectively referred to as “multipliedsignal.”

The current-voltage converting circuit 123 converts the multipliedsignal into the voltage to generate a multiplied voltage signal.

FIG. 4C illustrates an example of a circuit configuration of theimpedance element 421. The impedance element 421 employs the sameconfiguration as the impedance element 121 according to the firstembodiment except that the capacitor C3 is not provided.

The impedance element 421 has an inductance element L3 having one endconnected to the terminal i and having the other end connected to theterminal h and terminal j. The impedance element 421 directly connectsthe switching circuit 422 and voltage-current converting circuit 424,and connects the amplifying portion 410, switching circuit 422, andvoltage-current converting circuit 424 through the inductance elementL3. By this means, it is possible to provide both low AC and DCimpedances between terminals h and j, and provide a high AC impedanceand a low DC impedance between the terminals i and j. In addition,unlike the impedance element 121 described in the first embodiment, theimpedance element 421 does not need to be provided with the capacitor C3and ACwise grounded. This is because the amplifier 411-1 receives inputsof a normal phase signal and a reverse phase signal and therefore theterminal b where the normal phase signal and reverse phase signal crossis ACwise grounded.

The signal processing circuit 400 according to the present embodimentemploys the same operation as in a case where the controlling portion210 is applied to the configuration according to the third embodiment,except that a differential signal is used.

That is, the direct current I_(mixer) used to drive the amplifyingportion 411-1 is supplied to the voltage-current converter 424 throughthe impedance element 421. Further, the controlling portion 210 controlsthe potential of the connecting portion Q and the current flowing intothe switching circuit 422 to adjust the rate to the current flowing inthe voltage-current converter 424. Consequently, it is possible to driveall circuits according to a local signal of a low voltage withoutadditional power consumption, and realize a signal processing circuithaving good conversion gain and noise characteristics.

Further, it is also possible to use the direct current having been usedto drive the amplifier 411-1 to drive the amplifier 411-2 of a laterstage and, consequently, reduce power consumption.

In addition, in the present embodiment, although the configuration inwhich a controlling portion is applied to the signal processing circuit300 illustrated in FIG. 3 is changed to a differential circuit, thesignal processing circuit illustrated in FIGS. 1 to 3 may be configuredas a differential circuit.

<Fifth Embodiment>

Next, a signal processing circuit 500 according to a fifth embodimentwill be described. FIG. 5 is a view illustrating the signal processingcircuit 500 according to the fifth embodiment.

The signal processing circuit 500 has an amplifying portion 510 and afrequency converting circuit 120. Unlike the signal processing circuit100 according to the first embodiment, the amplifying portion 510 of thesignal processing circuit 500 has N amplifiers 111-1 to 111-N (N is aninteger of 3 or more). The other configurations are the same as those ofthe signal processing circuit 100 described in the first embodiment.

Each configuration of the amplifiers 111-1 to 111-N is the same as theconfiguration of the amplifier 111 described in the first embodiment.

The drain terminal of the kth (wherein k is an integer of 0<k<N)amplifier 111-k of the amplifiers 111-1 to 111-N is connected to thegate terminal of the (k+1)th amplifier 111-k+1, and the amplifier111-k+1 amplifies the amplified signal amplified in the amplifier 111-k. In terms of the alternating current, the amplifiers 111-1 to 111-N areconnected with each other in cascade.

The drain terminals of transistors provided in circuits of M amplifiers111-1 to 111-M (M is an integer equal to or more than 2 and equal to orless than N) among the amplifiers 111-1 to 111-N are respectivelyconnected to the first power source (Vdd). Further, the source terminalsof the transistors of the amplifiers 111-1 to 111-M are connected witheach other. Consequently, the transistors of the amplifiers 111-1 to111-M are DCwise connected in parallel in terms of the direct current.The amplifiers 111-1 to 111-M will be referred to as “upper amplifyingstage.”

The drain terminals of transistors provided in circuits of N-Mamplifiers 111-M+1 to 111-N (referred to as “lower amplifying stage”) ofthe amplifiers 111-1 to 111-N are connected with each other. The sourceterminals of transistors in the lower amplifying stage are connected tothe second power source potential (ground). Hence, the lower amplifyingstage is DCwise connected in parallel in terms of the direct current.The drain terminals of transistors in the lower amplifying stage arecommonly connected with source terminals of transistors in the upperamplifying stage, and therefore the upper amplifying stage and loweramplifying stage are DCwise connected in series in terms of the directcurrent signal.

Further, source terminals of transistors in the upper amplifying stageand drain terminals in the lower amplifying stage are connected to thevoltage-current converter 124 through the impedance element 121 of thefrequency converting circuit 120. In terms of the direct current signal,the voltage-current converter 124 is DCwise connected in series with theupper amplifying stage, and is DCwise connected in parallel with thelower amplifying stage.

In addition, as is clear from FIG. 5, the voltage-current converter 124is also DCwise connected in series with the switching circuit 122through the impedance element 121.

Hereinafter, currents flowing in the amplifying portion 510 will bedescribed in terms of a direct current and alternating current,respectively. First, the flow of an alternating current signal will bedescribed.

The amplifier 111-1 receives an input of an input signal which is analternating current signal through the terminal c. The amplifier 111-1amplifies the input signal, and generates a first amplified alternatingcurrent signal to output it from the terminal d. The first amplifiedalternating current signal is inputted to the terminal c of theamplifier 111-2. The amplifier 111-2 amplifies the first amplifiedalternating current signal, and generates the second amplifiedalternating current signal to output it from the terminal d. Then, theamplifiers 111-3 to 111-M also perform the same processing, and theamplifier 111-M generates an Mth amplified alternating current signal.The Mth amplified alternating current signal is inputted to the terminalc of the amplifier 111-M+1 in the lower stage. The amplifier 111-M+1amplifies the Mth amplified alternating current signal, and generates a(M+1)th amplified alternating current signal to output it from theterminal d. The (M+1)th amplified alternating current signal is inputtedto the terminal c of the amplifier 111-M+2. The amplifier 111-M+2amplifies the (M+1)th amplified alternating current signal, andgenerates a (M+2)th amplified alternating current signal to output itfrom the terminal d. Then, the amplifiers 111-M+3to 111-N also performthe same processing, and the amplifier 111-N generates an Nth amplifiedalternating current signal. The Nth amplified alternating current signalis inputted to the gate terminal of the transistor Ml of thevoltage-current converter 124 of the frequency converter 120 as theoutput signal of the amplifying portion 510 (referred to as “amplifiedsignal”). The subsequent flow of the alternating current signal is thesame as that of the first embodiment, and therefore, description thereofwill not be repeated.

Next, the flow of the direct current signal will be described.

Assume that direct currents I_(mixer), I_(amp1), I_(amp2), . . . andI_(ampM) flow between drain terminals and source terminals oftransistors in the upper amplifying stage. In this case,I_(mixer)+I_(amp1)+I_(amp2)+ . . . I_(ampM) are outputted from the upperamplifying stage. Part of the current (I_(mixer)) ofI_(mixer)+I_(amp1)+I_(amp2)+ . . . I_(ampM) is supplied to thevoltage-current converter 124, and the rest of the currents(I_(amp1)+I_(amp2)+ . . . I_(ampM)) flow between drain terminals andsources of transistors of 111-M+1 to 111-N in the lower amplifyingstage.

Similar to the first embodiment, the total amount of the direct currentsflowing into the switching circuit 122 is herein represented by I_(SW).This direct current I_(SW) is also inputted to the voltage-currentconverter 124 connected in series through the impedance element 121.Accordingly, the direct current I_(SW)+I_(mixer) flows into thevoltage-current converter 124.

With the signal processing circuit 500 and frequency converting circuit120 according to the present embodiment, the direct current I_(VI)flowing into the voltage-current converter 124 is greater, by an amountof the direct current I_(mixer) inputted from the amplifying portion510, than the direct current I_(SW) flowing into the switching circuit122. Consequently, it is possible to reduce noise produced in theswitching circuit 122 while maintaining the linearity of thevoltage-current converter 124, and decrease the amplitude of a localsignal required to drive the switching circuit 122.

Further, the signal processing circuit 500 according to the presentembodiment can also use the direct currents (I_(amp1)+I_(amp2)+ . . .I_(ampM)) having been used to drive amplifiers in the upper amplifyingstage in order to drive amplifiers in the lower amplifying stage, and,consequently, reduce power consumption of the amplifying portion 510.

FIRST MODIFIED EXAMPLE

Next, a signal processing circuit 600 according to a first modifiedexample of the fifth embodiment will be described. FIG. 6 is a circuitdiagram of the signal processing circuit 600 according to the firstmodified example of the fifth embodiment. As illustrated in FIG. 6, thesignal processing circuit 600 according to the first modified examplediffers from the signal processing circuit 500 according to the fifthembodiment in that the signal processing circuit 600 further has acontrolling portion 210.

The configuration and function of the controlling portion 210 are thesame as the configuration and function of the controlling portion 210 ofthe signal processing circuit 200 according to the second embodiment.However, the controlling portion 210 of the signal processing circuit600 differs from the signal processing circuit 200 according to thesecond embodiment in the condition of the reference voltage V_(ref) fordetermining the potential of the connecting portion Q. With the signalprocessing circuit 200 of the second embodiment, the condition of thereference voltage V_(ref) is a potential required to cause the amplifier111 to perform a desired operation. However, with the signal processingcircuit 600, the reference voltage V_(ref) is a potential required tocause the amplifiers 111-1 to 111-N to perform a desired operation.

To be more precise, with the signal processing circuit 200 according tothe second embodiment, the potential of the connecting portion Qdetermines the potential of the source terminal of the transistor of theamplifier 111, however, with the signal processing circuit 600, thepotential of the connecting portion Q determines potentials of sourceterminals of transistors of the amplifiers 111-1 to 111-M in the upperamplifying stage and determines potentials of drain terminals oftransistors of amplifiers 111-M+1 to 111-N in the lower amplifyingstage. Hence, the reference voltage V_(ref) is set to such a voltagethat the potential of the connecting portion Q is a potential of sourceterminals which allows transistors in the upper amplifying stage toperform a desired operation and a potential of the drain terminals whichallows transistors in the lower amplifying stage to perform a desiredoperation.

SECOND MODIFIED EXAMPLE

Next, a signal processing circuit 700 according to a second modifiedexample of the fifth embodiment will be described. FIG. 7 is a circuitdiagram of the signal processing circuit 700 according to the secondmodified example of the fifth embodiment. As illustrated in FIG. 7, theconfiguration of the signal processing circuit 600 according to thefirst embodiment is further configured as a differential circuit. Thedifferential circuit has been described in the fourth embodiment, andtherefore, description thereof will not be repeated. In addition, thesignal processing circuit 700 employs the same configuration as thesignal processing circuit described in the fourth embodiment except thatamplifiers are N amplifiers 411-1 to 411-N.

THIRD MODIFIED EXAMPLE

Next, a signal processing circuit 800 according to a third modifiedexample of the fifth embodiment will be described. FIG. 8 is a circuitdiagram of the signal processing circuit 800 according to the thirdmodified example of the fifth embodiment.

Although, with the signal processing circuit 500 according to the fifthembodiment, the amplifying portion 510 has a plurality of amplifiers111-1 to 111-N and a signal amplified by the kth amplifier is amplifiedby the (k+1)th amplifier, amplifiers 111-1 to 111-N of an amplifier 810according to the present modified example receive inputs of differentsignals and amplify different signals.

That is, the drain terminal of the kth (wherein k is an integer of0<k<N) amplifier 111-k of the amplifiers 111-1 to 111-N is not connectedto the gate terminal of the (k+1)th amplifier 111-k+1. The otherconfigurations are the same as those of the signal processing circuit500.

Hereinafter, the flow of an alternating current signal of the amplifyingportion 810 of the signal processing circuit 800 will be described. Thealternating current signal flowing into the frequency converting circuit120 and direct current signal flowing into the amplifying portion arethe same as those in the signal processing circuit 500, and therefore,description thereof will not be repeated.

The amplifiers 111-1 to 111-N−1 respectively receive inputs ofalternating current signals INPUT₁ to INPUT_(n−1) from outside throughthe terminal c. The amplifiers 111-1 to 111-N+1 respectively amplifyalternating current signals INPUT₁ to INPUT_(n−1) and generate amplifiedalternating current signals OUTPUT₁ to OUTPUT_(n-1) . The amplifiers111-1 to 111-N−1 respectively output the amplified alternating currentsignals OUTPUT₁ to OUTPUT_(n−)to outside through the terminal d.

With the example of FIG. 8, the amplifier 111-N amplifies the inputsignal INPUT_(N) inputted from the terminal c, and generates anamplified signal. The amplifier 111-N outputs the amplified signal fromthe terminal d to the gate terminal of the transistor M1 of thevoltage-current converter 124.

Although an example has been described with the above example where allamplifiers 111-1 to 111-N respectively amplify different signals, aconfiguration where some of amplifiers are connected in cascade andsequentially amplify input signals may be employed. That is, aconfiguration where the drain terminal of the kth (wherein k is aninteger of 0<k<N) amplifier 111-k of some of the amplifiers 111-1 to111-N is connected to the gate terminal of the (k+1)th amplifier111-k+1.

Further, although a configuration has been described with the example inFIG. 8 where the amplified signal generated by the amplifier 111-N isinputted to the voltage-current converter 124, amplified alternatingcurrent signals generated by the other amplifiers 111-1 to 111-N−1 maybe used as inputs of the voltage-current converter 124.

FOURTH MODIFIED EXAMPLE

Next, a signal processing circuit 900 according to a fourth modifiedexample of the fifth embodiment will be described using FIG. 9.

The signal processing circuit 900 differs from the signal processingcircuit 500 in that the upper amplifying stage and lower amplifyingstage are not connected in cascade in a case of the alternating currentsignal. Further, the signal processing circuit 900 differs from thesignal processing circuit 500 according to the fifth embodiment in thatthe signal processing circuit 900 amplifies the input signal in thelower amplifying stage and amplifies, in the upper amplifying stage, themultiplied voltage signal which is an output of the frequency convertingcircuit 920. Hereinafter, these will be described in detail.

First, the configuration of an amplifying portion 910 will be described.With the amplifying portion 910, the upper amplifying stage and loweramplifying stage are not connected in cascade in the case of thealternating current signal. To be more specific, the drain terminal ofthe amplifier 111-M in the upper amplifying stage and the gate terminalof the amplifier 111-M+1 in the lower amplifying stage are notconnected.

All amplifiers 111-1 to 111-M in the upper stage are connected incascade. That is, the drain terminal of the kth (wherein k is an integerof 0<k<M) amplifier 111-k is connected to the gate terminal of the(k+1)th amplifier 111-k+1. Further, all amplifiers 111-M+1 to 111-N inthe lower stage are connected in cascade. That is, the drain terminal ofthe kth (wherein k is an integer of M<k<N) amplifier 111-k is connectedto the gate terminal of the (k+1)th amplifier 111-k+1.

Further, the terminal c of the amplifier 411-1 in the upper amplifyingstage is connected to the terminals f and g of the current-voltageconverting circuit 123 through an impedance element 930. Further, theterminal c of the amplifier 411-M+1 in the lower amplifying stagereceives an input of an input signal. The terminal d of the amplifier411-N in the lower amplifying stage is connected to the voltage-currentconverter 424.

Next, the flow of an alternating current signal will be described. Theflow of the direct current is the same as that of the signal processingcircuit 500, and therefore, description thereof will not be repeated.

The input signal is inputted through the terminal c of the amplifier411-M+1 in the lower stage. The amplifier 411-M+1 to amplifier 411-Namplify the input signals, and output amplified signal from the terminald of the amplifier 411-N. The amplified signal is outputted to thevoltage-current converter 124. Next, the amplified signal is convertedinto a multiplied voltage signal by the frequency converting circuit920. Subsequently, the multiplied voltage signal is inputted to theterminal c of the amplifier 411-1 in the upper stage through theimpedance element 930. The amplifier 411-1 to amplifier 411-M in theupper stage amplify the multiplied voltage signals and output theamplified multiplied voltage signals from the terminal d of theamplifier 411-M. That is, the signal processing circuit 900 according tothe fourth modified example employs a configuration where the amplifiers411-M+1 to 411-N in the lower stage operate as an amplifying portion ata stage before the frequency converting circuit 920, and the amplifiers411-1 to 411-M in the upper stage operate as an amplifying portion inthe later stage.

In addition, a configuration may be employed where the amplifiers 411-1to 411-M in the upper stage operate as the amplifying portion at thestage before the frequency converting circuit 920, and the amplifiers411-M+1 to 411-N in the lower stage operate as an amplifying portion.

<Sixth Embodiment>

Next, a receiver 1500 according to a sixth embodiment will be describedusing FIG. 10.

The receiver 1500 according to the sixth embodiment uses a signalprocessing circuit 400 as a LNA (Low-Noise Amplifier) built-in mixer,and has an orthogonal demodulation function of the superheterodynesystem.

The receiver 1500 has an antenna 1510 which receives a high frequencysignal (input signal); a first local oscillator 1520 which generates afirst local signal; the signal processing circuit 400 which amplifies aninput signal, generates an amplified signal, and generates a multipliedvoltage signal by multiplying the amplified signal by a first localsignal; a second local oscillator 1560 which generates a second localoscillation signal; a phase shifter 1570 which generates a third localoscillation signal obtained by shifting the phase of the second localoscillation signal by 90 degrees; a mixer 1530-1 which multiplies themultiplied voltage signal by a second local signal and generates a firstdemodulated signal; a mixer 1530-2 which multiplies the multipliedvoltage signal by a third local signal and generates a seconddemodulated signal; a lowpass filter 1540-1 which extracts a signal of adesired band from the first demodulated signal and generates a firstextracted signal; a lowpass filter 1540-2 which extracts a signal of adesired band from the second demodulated signal, and generates a secondextracted signal; a variable gain amplifier 1550-1 which amplifies thefirst extracted signal and generates a first output signal; and avariable gain amplifier 1550-2 which amplifies the second extractedsignal and generates a second output signal. The first output signal andsecond output signal are outputted to a signal processing portion whichis not illustrated. The signal processing portion performs signalprocessing such as A/D conversion of the output signals.

As illustrated in FIG. 4, the signal processing circuit 400 employs aconfiguration including the amplifying portion 410 which amplifies aninput signal and generates an amplified signal, and a frequencyconverting circuit 420 which generates a multiplied voltage signal fromthe amplified signal.

The receiver 1500 according to the present embodiment can realize amixer which can be driven by a local signal of a low voltage withoutadditional power consumption, using the signal processing circuit 400 asthe LNA (Low-Noise Amplifier) build-in mixer, and provide goodconversion gain and noise characteristics.

Although the receiver 1500 employs the configuration using the signalprocessing circuit 400 with the present embodiment, any one of thesignal processing circuits 100 to 900 may be used.

Further, although the receiver 1500 has an orthogonal demodulationfunction of the superheterodyne system with the present embodiment, theconfiguration of the receiver is not limited to this.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andsprit of the inventions.

What is claimed is:
 1. A frequency converting circuit which generates amultiplied signal obtained by multiplying a local signal by an amplifiedsignal generated by an amplifying portion comprising a first transistorhaving a drain terminal connected to a first power source potential, thefrequency converting circuit comprising: a converter which comprises atleast one second transistor having a gate terminal connected to theamplifying portion and which converts the amplified signal inputted tothe gate terminal into a current signal; a switching circuit whichcomprises at least two third transistors whose source terminals areconnected to each other and which multiplies the current signal by thelocal signal and generates the multiplied signal; and an impedanceelement which comprises a first terminal connected to a source terminalof the first transistor, a second terminal connected to a drain terminalof the at least one second transistor and a third terminal connected tothe source terminals of the at least two third transistors, whichsupplies a first direct current from the source terminal of the firsttransistor and a second direct current from the source terminals of theat least two third transistors to the drain terminal of the at least onesecond transistor, wherein impedance of the impedance element comprisesa high AC impedance between the first terminal and the second terminal.2. The frequency converting circuit according to claim 1, furthercomprising a controlling portion which controls the second directcurrent such that a first potential of a connecting portion between theamplifying portion and the first terminal of the impedance elementbecomes a desired potential.
 3. The frequency converting circuitaccording to claim 1, further comprising a controlling portion whichcompares a first potential of a connecting portion between theamplifying portion and the first terminal of the impedance element witha reference voltage, and supplies a voltage to the third transistorssuch that the first potential matches the reference voltage.
 4. Thefrequency converting circuit according to claim 1, wherein: theamplified signal comprises a normal phase amplified signal and a reversephase amplified signal; the at least one second transistor of theconverter comprises: a normal phase second transistor which generates anormal phase current signal from the normal phase amplified signal; anda reverse phase second transistor which generates a reverse phasecurrent signal from the reverse phase amplified signal, and wherein thecurrent signal comprises the normal phase current signal and the reversephase current signal; and the switching circuit comprises: twonormal-phase third transistors which generate normal phase multipliedsignals from the normal phase current signal and which are connected inparallel; and two reverse-phase third transistors which generate reversephase multiplied signals from the reverse phase current signal and whichare connected in parallel, and wherein the multiplied signal comprisesthe normal phase multiplied signal and the reverse phase multipliedsignal.
 5. A receiver comprising: an antenna which receives an inputsignal; an amplifying portion which amplifies the input signal andgenerates an amplified signal; and a frequency converting circuitaccording to claim 1 which multiplies the amplified signal by a localsignal and generates a multiplied signal.
 6. A signal processing circuitcomprising: an amplifying portion for generating an amplified signal,the amplifying portion comprising: M (where M is an integer equal to ormore than 2 and equal to or less than N) amplifiers which each comprisea first transistor, source terminals of the respective first transistorsbeing connected together and drain terminals of the respective firsttransistors being connected to a first power source potential; and N-M(N is an integer equal to or more than 3) amplifiers which each comprisea second transistor, drain terminals of each of the respective secondtransistors being connected to the connected-together source terminalsof the first transistors and source terminals of the respective secondtransistors being connected to a second power source potential; aconverter which comprises at least one third transistor having a gateterminal connected to the amplifying portion and which converts theamplified signal supplied to the gate terminal; a switching circuitwhich comprises at least two fourth transistors whose source terminalsare connected to each other and which multiplies the current signal bythe local signal and generates a multiplied signal; and an impedanceelement which comprises a first terminal connected to theconnected-together source terminals of the first transistors, a secondterminal connected to a drain terminal of the at least one thirdtransistor and a third terminal connected to the connected-togethersource terminals of the at least two fourth transistors, which suppliesa first direct current from the connected-together source terminals ofthe first transistors and a second direct current from theconnected-together source terminals of the fourth transistors to thedrain terminal of the at least one third transistor, wherein impedanceof the impedance element comprises a high AC impedance between the firstterminal and the second terminal.
 7. The signal processing circuitaccording to claim 6, wherein a drain terminal of a kth (k is an integerof 0<k<N) transistor of the first and second transistors of theamplifying portion is connected to a gate terminal of a (k+1)thtransistor of the first and second transistors; and the (k+1)thamplifier amplifies an amplified alternating current signal amplified bythe kth amplifier.
 8. A receiver comprising: an antenna which receivesan input signal; and the signal processing circuit according to claim 6.9. A frequency converting circuit comprising: an amplifying portionwhich amplifies an input signal; a converter which converts an amplifiedsignal, which is amplified by the amplifying portion, into a currentsignal; an impedance element which connects the amplifying portion andthe converter with a high AC impedance and which supplies to theconverter a direct current from the amplifying portion; and a switchingcircuit which comprises a pair of transistors connected in parallel,which is connected with the converter through the impedance element, andwhich generates a multiplied signal by multiplying a local signal by thecurrent signal.
 10. The frequency converting circuit according to claim9, wherein the amplifying portion comprises a transistor having a drainterminal connected to a first power source.
 11. The frequency convertingcircuit according to claim 9, wherein the amplifying portion comprisestwo or more amplifiers connected in cascade.
 12. The frequencyconverting circuit according to claim 11, wherein the amplifyingportion, the switching circuit, and the converter are operable in normaland reverse phases.
 13. The frequency converting circuit according toclaim 10, wherein: the amplifying portion comprises a plurality ofcascade-connected first amplifiers which are connected to a first powersource potential, and a plurality of cascade-connected second amplifierswhich are connected to a second power source potential; and transistorscomprising the plurality of first amplifiers have drain terminalsconnected to the first power source potential and have source terminalscommonly connected, and transistors comprising the plurality of secondamplifiers have drain terminals commonly connected and have sourceterminals connected to the second power source potential.
 14. A receivercomprising: an antenna which receives an input signal; the frequencyconverting circuit according to claim 12.